Over the last few years III-V nitrides like GaN have drawn a lot of interest as promising materials for high-temperature and high-power electronics. Future high-efficiency power converters require fast switching, low conduction loss devices that can handle high voltages. GaN is a good candidate for voltages up to 1 kV and shows excellent switching behaviour in Schottky diodes and in high-electron mobility transistors (HEMTs). Thanks to the advancements in GaN-on-Si epitaxy, the semiconductor industry is now actively combining III-V specific device expertise with low-cost high-volume Si main-stream production facilities.
One of the key considerations for main-stream Si compatibility is the choice of metals used and as the technology advances, more stringent demands will be made on the reproducibility, uniformity, thermal stability, and high temperature operation of GaN-based semiconductor devices.
Most ohmic contacts on GaN/AlGaN heterostructures are based on Ti/Al-based metallization schemes. Titanium creates nitrogen vacancies in the underlying GaN by forming TiN, which enables electrons to tunnel to the 2-dimensional electron gas (2DEG) underneath the AlGaN. Aluminium is included to react with Ti to prevent oxidation of the Ti. On top of the Al, gold is commonly used as the bulk metal, often separated by a diffusion barrier. Common metallization structures include Ti/Al/Ti/Au, Ti/Al/Ni/Au and Ti/Al/Pt/Au.
However, gold not only is expensive but also is incompatible with mainstream silicon-based semiconductor device manufacturing processes. Therefore, to be able to process GaN/AlGaN HEMTs on GaN-on-Si substrates in a standard Si fab, gold has to be eliminated from the process and replaced by a main-stream Si-compatible metal.
Such semiconductor devices may further or alternatively include a Schottky contact, which may comprise a nickel layer in contact with the at least one active layer of the semiconductor device. This also is not without problems. For instance, when aluminium is used as the metal of choice in the backend, aluminium can diffuse into the nickel, which negatively affects the properties of the Schottky contact.
EP 2 416 364 A2 discloses a GaN-based semiconductor device having a Schottky contact including a first metal contact layer and a second Schottky metal contact layer disposed on the first metal contact layer. The second Schottky metal contact layer has a lower work function than the first metal contact layer. The first metal contact layer preferably includes nickel and the second Schottky metal contact layer may be selected from Pd, TiW interlayer, Pt, Al, Ti, Mo, Au or a combination thereof. However, it has been found that the suggested second Schottky metal contact layer does not satisfactorily address the aforementioned problems.